1. Field of the Invention
The present invention relates to a high-frequency discharge lamp comprising a high-frequency discharge tube for emitting light of discharge by plasma generated by high-frequency electromagnetic waves transmitted by a coaxial waveguide constructed of an internal conductor and an external conductor.
2. Background Art
FIGS. 14 and 15 are a conventional high-frequency discharge lamp shown in the following Patent Reference 1, and comprise a coaxial waveguide 1 for high-frequency electromagnetic wave transmission constructed of an internal conductor 2 and an external conductor 3, and a discharge tube 4 which is attached to the top of the waveguide 1 and emits light of discharge by plasma generated by electromagnetic waves transmitted by the waveguide 1 and has an outside diameter almost equal to an outside diameter of the waveguide 1. That is, the top of the waveguide 1 for transmitting high-frequency electromagnetic waves generated by a sending part is provided with an electromagnetic wave irradiation part 6 comprising an internal conductor 6a and an external conductor 6b respectively connected to the internal conductor 2 and the external conductor 3 of the waveguide 1. Electromagnetic waves (a high-frequency electric field generated by the electromagnetic wave irradiation part 6) are irradiated from an annular top plate part 6b1 of the external conductor 6b and a disk-shaped top part 6a1 of the internal conductor 6a opposed with an annular slit 6c sandwiched between the parts. High-density plasma is generated inside the discharge tube 4 and a light emission substance of the inside of the discharge tube 4 is evaporated and excited and emits light.
Since an electrode is not disposed inside discharge space of the discharge tube 4, there is no heat loss from the electrode and light emission efficiency (lumen/watt) of the discharge tube improves accordingly and it is unnecessary to consider a reaction between a conductor assembly and an enclosure substance (a metal halide) of the inside of the discharge space, so that a light emission substance suitable to improve the light emission efficiency can be used.
[Patent Reference 1] JP-A-2005-228520
However, in the conventional art described above, when a pressure of a rare gas for starting enclosed with the inside of the discharge tube 4 (discharge space) is increased, a density of plasma generated in the discharge space becomes high and the amount of light increases, but a starting onset voltage rises with an increase in the pressure of the rare gas and starting (lighting) cannot be performed, so that an enclosure pressure of the rare gas is set at less than 1 atmospheric pressure at room temperature and as a result of this, the plasma density does not increase and a sufficient amount of light cannot be obtained.
Further, since electromagnetic waves are guided to the discharge space through a bottom wall of the discharge tube 4, Joule loss by heating of the bottom wall is large and the bottom wall of the discharge tube 4 is arranged so as to make contact with a top surface (the disk-shaped top part 6a1 of the internal conductor 6a and the annular top plate part 6b1 of the external conductor 6b) of the electromagnetic wave irradiation part 6 with a large heat capacity, so that loss by heat conduction is large and light emission efficiency does not increase.
Also, the discharge tube 4 is a bottomed cylindrical body with a large surface area and has large loss of heat radiation from the tube surface and has less efficiency of light emission and further is not realistic in the case of considering a process of manufacturing the discharge tube.